Electron discharge device and circuit



Jan. 10, 1967 M. E. u uG 3 ,297,900

ELECTRON DISCHARGE DEVICE AND CIRCUIT Filed April 5, 1963 4 Sheets-Sheet 1 INVENTOR. MEHMET E. ULUG HIS A ORN Y Jan. 10, I M. E- ULUG ELECTRON DISCHARGE DEVICE AND CIRCUIT Filed April 5, 1965 4 Sheets-Sheet 2 FIG.

INVENTOR.

MEHMET E. ULUG HIS ATTO NEY Jan. 10, 1967 M. E. ULUG 3,297,900

ELECTRON DISCHARGE DEVICE AND CIRCUIT Filed April 5, 1963 4 Sheets-Sheet 5 FIG. 5

OUT UT s20 3 j' AG-c 93 if... I I 9| v INPUT 92 w ji i FIG. 6

INVENTOR. MEHMET E. ULUG HIS TTO N EY Jan. 10, 1967 M. E. ULUG ELECTRON DISCHARGE DEVICE AND CIRCUIT 4 Sheets-Sheet 4 Filed April 5, 1963 gall-l ly FIG. 8

INVENTOR.

MEHMET E. ULUG United States Patent Ofiiice 3,297,990 Patented Jan. 10, 1967 3,297,900 ELECTRON DISCHARGE DEVICE AND CIRCUIT Mehmet Esin Ulug, Etobicoke, Ontario, Canada, assignor to Canadian General Electric Company, Limited, Toronto, Ontario, Canada, a corporation of Canada Filed Apr. 3, 1963, Ser. No. 270,320 2 Claims. (Cl. 313295) This invention relates to a novel electron discharge device or tube and circuits employing such devices. More particularly the invention relates to a novel tube of the pentode type, in which both the control grid and the suppressor grid can be used to modulate the plate current.

Pentode tubes are generally of two types, sharp cut-off pentodes and remote cut-oft pentodes. These are well known and need not be described in detail. The sharp cut-01f pentodes have a high gain, but generally are not suitable for use in an amplifying circuit to which AGC voltage is applied. Remote cut-off pentodes are generally used in amplifiers to which AGC voltage is applied, but they suffer several disadvantages. The control grid of a remote cut-off pentode is wound with a varying pitch, with the turns in the middle of the grid being widely spaced relative to the turns at the ends. This results in a loss of gain in the amplifying stage. Moreover the density of the stream of electrons passing through the control grid varies over the cross-section of the stream. As a consequence,

parts of the screen-grid may be overheated when the tube conducts heavily.

In a conventional amplifying stage employing a remote cutotI pentode, the automatic gain control voltage is applied to the control grid of the pentode. As this voltage varies, the space charge between the cathode and the control grid varies in magnitude. This means that the input capacitance of the tube varies, increasing as the space charge increases. Associated with this problem is the additional problem posed by electrons which fail to pass the suppressor grid and which return to the region of the cathode to oscillate in the space between the cathode and the control grid. This has the effect of loading the control grid and hence decreasing the input impedance.

Such changes in the parameters of the input circuit of the amplifying stage prevent the resonant frequency of the input circuit from being constant. However, in IF. amplifying stages, it is highly desirable that the resonant frequency of various circuits including such input circuits be held as nearly constant as possible, in order to avoid the distortion produced in the output signal by changes in the resonant frequency of the circuits.

The foregoing, and other disadvantages of conventional pentodes and circuits are substantially avoided by the present invention comprising a novel tube. The novel tube comprising a cathode and an anode, defining therebetween at least one main path for electrons emitted from the cathode. Between the cathode and the anode are, in order, a control grid, a screen grid, and a suppressor grid. The control grid and the screen grid are both closely wound with a constant pitch. The tube therefore has a high transconductance, the cathode operates as a high efliciency, and overheating of parts of the screen grid is avoided. The suppressor grid is preferably also closely wound with a constant pitch, but it may, if desired, be wound with a varying pitch.

In the novel pentode, the construction of the grids gives a high gain'to the tube, but it means that many electrons will not be able to pass the suppressor grid in order to reach the anode. It is desirable to prevent such electrons from returning to the region between the cathode and the control grid. Means are provided to prevent such return of these electrons: these means comprise, firstly, electron-defiecting means cooperating with the suppressor grid, and secondly, electron-collecting means cooperating with the screen grid. The said deflecting means may comprise simply a deformation in the contour of the suppressor grid, the deformation preferably being in the form of a V-shaped projection pointing toward the cathode and in the main path of the current. The said collecting means may comprise auxiliary electrodes in the form of plates electrically connected to the screen grid and positioned laterally of the main path of the current. The deflecting means act to deflect slow electrons toward the collecting means, which capture these electrons and thus prevent their return to the region between the cathode and the control grid.

By the above-described deflecting and collecting means, the input capacitance of the tube is kept substantially constant and the input impedance is kept large. These features and others, of my novel tube are utilized to advantage in IE. (intermediate frequency) amplifiers in television receivers.

In drawings which illustrate embodiments of the invention,

FIGURE 1 is a schematic representation of a transverse crosssection of a preferred embodiment of my novel tube;

FIGURE 2 is a schematic representation of a transverse cross-section of another embodiment of my novel tube;

FIGURE 3 is a schematic circuit diagram of a typical amplifying stage of the prior art;

FIGURE 4 is a graph used explaining the action of the circuit of FIGURE 3;

FIGURE 5 is a schematic circuit diagram broadly illustrating certain applications of the invention;

FIGURE 6 is a schematic circuit diagram of a single stage of amplification employing my novel tube;

FIGURE 7 is a schematic representation of a transverse cross-section of still another embodiment of my novel tube; and

FIGURE 8 is a graph used in explaining the action of the circuit of FIGURE 6.

In FIGURE 1, reference numeral 8 indicates the glass envelope of an evacuated electron discharge device or tube 9. A metallic shield It) is provided inside envelope 8. Shield 10 provides electrostatic shielding for the internal structure of the tube. The shield and other electrodes of the tube are connected to pins (not shown) at the base of the tube according to techniques well known in the art.

Shield 10 may be a conventional electrostatic shield, such as is well known in the art. An electron-emissive cathode 11 is located axially in tube 9. Cathode 11 is preferably of the type which is indirectly heated, the heater being indicated in FIGURE 1 at 11. Cathode 11 is indicated as being rectangular, and is to be understood as preferably having electron-emissive coatings on at least the two sides represented by the longer sides of the rectangle.

Surrounding and spaced from cathode 11 is a first or control grid 12 wound on and supported by rods 13. A second or screen grid 14 in turn surrounds and is spaced from control grid 12. Screen grid 14 is wound on and supported by rods 15. A third or suppressor grid 16 in turn surrounds and is spaced from screen grid 14, and is wound on and supported by rods 17. The several grids are coaxial with cathode 11, the various supporting rods being collinear with the long axis of the rectangule of the cathode. Such modes of constructions are well known in the art.

Included inside shield 10 is an anode 18, indicated as being in two parts in FIGURE 1. The two parts are to be understood as being electrically connected together by means not shown, such as a conductive metallic ribbon for example. The anode 18 of course is separated from cathode 11 by the three grids. The two parts of the anode face the longer sides of the cathode and are so situated that the line joining the midpoints-of the anodes would substantially bisect the cathode and be perpendicular to the longer sides thereof. The cathode and the two parts of the anode thus define two main paths for electrons emitted by the cathode. Each part of anode 18 has a humped portion 21 in the middle thereof, opposite the portions 19 of the suppressor grid, was to keep the spacing between the anode and the suppressor grid roughly constant.

Control grid 12 and screen grid 14 are closely wound, with a constant pitch. Suppressor grid 16 may be wound with either a constant pitch or a varying pitch. In a preferred embodiment the suppressor grid is also closely wound with a constant pitch. With such grids, the tube has a high transconductance, but many electrons, particularly slow electrons, are unable to pass the suppressor grid to reach the anode. Ordinarily many of these electrons would tend to return to the region between the control grid and the cathode, to oscillate there with deleterious effects in the input circuit of the tube. To overcome this difliculty, means are provided to remove such electrons from the main paths of the current and to prevent their returning to the region of the cathode. These means include the V-shaped portions 19 of the suppressor grid and the auxiliary electrodes 20. V-shaped portions 19 of grid 16 lie in the main paths of the current and project toward the cathode. The portions constitute defiecting means to deflect the majority of slow electrons toward the auxiliary electrodes 20. The latter capture these deflected electrons and thus remove them from the main paths of the current. Electrodes 20 also act as electrostatic shields between the anode 18 and support rods 13, and thus reduce the capacitance between the anode and the control grid. The auxiliary electrodes 20 may be welded to the rods 15, since they are preferably at about the same electrical potential as screen grid 14.

FIGURE 2 is similar to FIGURE 1 and illustrates some modifications of the tube shown in FIGURE 1. In the tube shown in FIGURE 2, the suppressor grid 30 does not have the V-shaped indentations 19. Instead, the electron-deflecting means are provided by separate rods 31, having each a sharp-edged portion 32 directed toward the cathode. Defiecting rods 31 may of.course have cross-sectional shapes other than the triangular one indicated, but in any case, the portion of the deflecting means facing the cathode should preferably have such a shape as indicated in both FIGURES 1 and 2.

Rods 31 preferably operate at the same electrical potential as suppressor grid 30, and hence may be electrically connected thereto. For example, rods 31 could be welded to the suppressor grid, and thus serve as additional supporting means for the grid, as well as serving as electrondeflecting means.

In the tube of FIGURE 2, anode 33 is shown in two parts, similarly to that of the tube of FIGURE 1. It is to be understood that the two parts of anode 33 are electrically connected, similarly to anode 18. Anode 33 does not require the humped portions 21 in order to maintain constant the spacing between the anode and the suppressor grid.

In FIGURE 2 the electron-collecting means 20 are shown separated from screen grid 14 as an additional option. However, if so separated, they may be electrically connected to the screen grid, as by a conducting ribbon for example, since preferably they operate at the same electrical potential as the screen grid.

FIGURE 7 shows a modification of the tube of FIG- URE 1. The support rods 17 of the suppressor grid of the tube in FIGURE 1 have been omitted from the tube of FIGURE 7. In FIGURE 7 the suppressor grid 40 is preferably less oval than suppressor grid 16 of FIG- URE l, for greater rigidity. Suppressor grid 40 is fixed to and supported by rods 41, which also serves as the electron-deflecting means, being situated in the main paths of the current and shaped similarly to the portions 19 of FIGURE 1. The anode in the tube of FIGURE 7 may be similar to anode 33 of FIGURE 2.

Other modifications of the novel tube will readily occur to skilled in the art. For example, the suppressor grid could be constructed in two parts in accordance with well known techniques, the two parts being, of course, electrically connected together.

FIGURE 3 illustrates the circuit of a conventional amplifying stage of the prior art, such as may be used for example in the LF. stages of a television receiver. The circuit employs a conventional remote cut-off pentode 50. The cathode 51 is connected to ground through a resistor 52 of low value. Supressor grid 53 is directly connected to the cathode. The control grid 54 is connected to an input terminal 55 of a pair of input terminals, the other one of which, terminal 56 is grounded. A source (not shown) of AGC voltage is applied to a terminal 57 which is connected to control grid 54 by resistors 58 and 59 in series. The junction of resistors 58 and 59 is decoupled to ground by way of a capacitor 60.

The output circuit comprises an output transformer 61 having a primary winding 62 and a secondary winding 63. Primary winding 62 has one end connected to the anode or plate 68 of tube 50, ad the other end connected to a high tension terminal 64 through a decoupling resistor 65. The screen grid 69 of the tube is connected directly to the junction of winding 62 and resistor 65. The said junction is decoupled to ground through a decoupling capacitor 66.

Secondary winding 63 is connected to output terminals 67, where an amplified reproduction of the input signal is obtained. This'completes the description of the circuit of FIGURE 3, the operation of which can be briefly described with reference to FIGURE 4.

FIGURE 4 shows a graph of the plate or anode current 1,, of tube 50 plotted against the control grid voltage V With an input signal of small amplitude, such as signal (a), the AGC voltage holds the control grid of tube 50 only slightly negative, and the variation in plate current is indicated by curve A. With an input signal of large amplitude, such as signal (b), the AGC voltage sends control grid 54 much more negative, and the plate current then varies as indicated by curve B. Curve A fairly faithfully reproduces the shape of signal (a), but curve B shows distortion on the negative-going portions. This disadvantage of conventional amplifiers using remote cutoff pentodes is well known. It will be shown below how this particular disadvantage is overcome by the present invention, in connection with the description of FIGURES 6 and 8.

FIGURE 5 indicates in a general way how my novel tube may be used in various types of circuit, such as amplifiers, oscillators, mixers, etc. Reference numeral 70 indicates a pentode constructed according to the instant invention, as described in FIGURE 1 for example. Various elements of tube 70 are therefore labelled with the reference numerals used in FIGURE 1 to indicate corresponding elements. Electrostatic shield 10 is indicated at 71 as being grounded, ground being taken as a common reference terminal or level of voltage.

A feature of my novel tube, not shown in FIGURE 1, is indicated in FIGURE 5. The cathode 11 has two cathode leads 72 and 73 attached thereto, at respective opposite ends of the cathode. The provision of two cathode leads increases the input impedance of the tube. The cathode leads'72 and 73 may be electrically connected together at the pins of the tube, as indicated in FIGURE 5. The term electrically connected means connected without resistance.

If it is desired to bias the cathode of tube 70 positive with respect to ground, as in an amplifier for example, a bias resistor 74 may be connected between ground and the junction of cathode leads 72 and 73. A bypass capacitor 75 is preferably connected in parallel with bias resistor 74 in circuits using my novel tube.

It will be noted that no bypass capacitor is provided in the conventional amplifier of FIGURE 3. In such an amplifier it is desirable to have some negative feedback from the cathode resistor, to offset changes in the input capacitance, and hence the bypass capacitor is omitted. However, in an amplifier using my novel tube, the input capacitance remains substantially constant, Hence no negative feedback from the cathode resistor is required, and a bypass capacitor is accordingly provided. The presence of the bypass capacitor 75 improves the transconductance of the tube, and is one of the advantages of the present invention.

The control grid 12 of tube 70 is shown connected through an input circuit 76 to ground. The particular type of input circuit used would depend on the particular application of the circuit of FIGURE 5. It is within the scope of those skilled in the art to provide the details of the various types of input circuits.

An output circuit 77 is connected between the anode 18 of tube 70 and one end of a decoupling resistor 78. The other end of resistor 78 is connected to a high-tension terminal 79. The junction of resistor 78 and output circuit 77 is decoupled to ground through a capacitor 80. As with input circuit 76, it is within the scope of those skilled in the art to provide the details of particular output circuits.

The screen grid 14 of tube 70 may be connected to the junction of resistor 78 and capacitor 80, this junction being a decoupled source of high tension for the tube. The connection may be direct, or it may be made through a screen-grid resistor 81 shown in FIGURE 5, if it is desired to limit the screen-grid current. When the screengrid resistor is present, the screen grid is decoupled to ground through a capacitor 82. The electron-collecting electrodes are indicated in FIGURE 5, one of them being labelled 20. The deflecting means are indicated at 19.

Tube 70 may be used either with or without AGC. If used without AGC, the suppressor grid 16 is held at a fixed position potential of several volts, and a high transconductance is obtained. If the tube is used with AGC, the suppressor grid may be connected as shown in FIGURE 5.

An AGC terminal 83 is provided to which AGC voltage may be applied. Terminal 33 is decoupled to ground through resistor 84 and capacitor 85 connected in series. The junction of resistor 84 and capacitor 85 may be connected through an inductance 86 to suppressor grid 16. Inductance 86 may be omitted in particular applications. When used, as in an amplifying circuit for example, its main function is to enable the gain of the tube to be increased Without having to raise the potential of the suppressor grid above zero, relative to the cathode. This function is important when the circuit of FIGURE is used as an amplifier. In such a case the inductance 86 is tuned to resonate with the capacitance between ground and the supressor grid. For example, if the circuit is used as an amplifier in the IF. (intermediate frequency) stages of a television receiver, to amplify frequencies of about 45 mc./s., the inductance 86 will resonate at about 60 mc./s. The quadrature grid current of the IR spectrum then causes the voltage on the suppressor grid to be almost in phase with the input signal on the control grid. This results in from 3 to 5 db (decibels) increase in gain without having to raise the AGC voltage above zero.

Suitable regenerative coupling may be provided between the input circuit and the output circuit. FIGURE 5 then would represent an oscillator. To function as, say, a mixer, there would be no coupling required between the input and the output. The input circuit 76 could then comprise for example, the secondary windings of two transformers, connected in series. The primary windings could have signals of differing frequencies therein. The output circuit 77 could then comprise an output transformer tuned to either the difference or the sum of the two input signals.

FIGURE 6 shows the circuit of a single stage of amplification suitable for use in the IF. stages of a television receiver. FIGURE 6 is similar to FIGURE 5 and is intended to represent a particular embodiment of the general circuit of FIGURE 5. The tube of FIGURE 6 is again of the same type as the tube illustrated in FIGURE 1. The cathode circuit and the suppressor-grid circuit are the same as those shown in FIGURE 5. The cathode resistor may be about 22 ohms, and the AGC decoupling resistor about 220 ohms. All of the capacitors shown in FIGURE 6 may be about 820 micro-microfarads.

The input circuit comprises input terminals 91, one of which is grounded and the other of which is connected to the control grid. A resistor 92 connects the control grid to ground. The resistance of resistor 92 need not be high; a value of about 2000 or 3000 ohms is suflicient. Such an intermediate value of resistance of the controlgrid resistor prevents instability of the amplifier, in case of the presence of some gas in the tube, or in case of emission of electrons from the control grid. The fact that resistor 92 can have such a value is an advantage accruing from the characteristics of my novel tube.

In tube 90 an inductance 93 is indicated. This inductance represents the inductive reactance of the lead connecting the screen-grid to its pin at the base of the tube. The reactance is such as to produce some slight regeneration in the input circuit without introducing instability. Such screen-grid regeneration is Well known in the art.

The screen-grid circuit external to tube 90 is the same as that of FIGURE 5. The screen-grid resistor may have a value of about 3300 ohms.

The H.T. circuit is the same as that in FIGURE 5. The H.T. decoupling resistor may have avalue of about 220 ohms. The high tension applied to the HT. terminal may be about 135 volts. A higher voltage, for example 270 volts, may be used if desired or necessary to accommodate input signals of large amplitude. If 270 volts is used, then the'cathode resistor may be about 47 ohms and the screen-grid resistor about 27,000 ohms.

The output circuit of FIGURE 6 is shown as comprising an output transformer similar to and similarly connected as that of the amplifier of FIGURE 3. It may be tuned, by means not shown, to the frequency of the incoming signals.

Owing to the special construction of my novel tube, and the circuit connections illustrated by way of example in FIGURE 6, the current in tube 90 remains substantially constant. Hence the control grid remains at a substantially constant negative voltage relative to the cathode. The bias on the control grid is therefore determined by the value of the cathode resistor, and is independent of the value of the AGC voltage. With this in mind, the operation of the amplifier of FIGURE 6 may be further briefly explained with reference to FIGURE 8.

FIGURE 8 is a graph showing the plate current of tube 90 plotted against the bias voltage on the control grid. As mentioned just above, the bias voltage is substantially constant, and therefore is substantially independent of the amplitude of the input signal within the range of amplitudes which the amplifier is designed to handle. Thus curves '(a) and (b) in FIGURE 8 indicate signals of respectively small and large amplitude. When signal (a) is impressed on the control-grid, the AGC voltage is such that the variation of plate current is described by the upper curve on the graph, and the form of the plate current is indicated by curve A.

The magnitude of the AGC voltage is, as usual, dependent on the amplitude of the incoming signal. Hence, when this signal is (b), a more negative AGC voltage is impressed on the suppressor grid. Consequently the plate current diminishes and the screen-grid current increases,

as more electrons are deflected by the deflecting means 19 to the collecting means 20. With signal (b) on the control grid, therefore, the variation of plate current is described by the lower curve 101 of FIGURE 8, and the form of the plate current is indicated by Curve B.

From the foregoing description it can be seen that the supressor grid largely determines the division of current between the plate circuit and the screen-grid circuit, accordance to the magnitude of the applied AGC voltage, without substantially altering the total current. In other words, the suppressor grid, including the deflecting means attached thereto or working in conjunction therewith, and the collecting means cooperating with the screen-grid, together constitute a current-switching means for directing at least a portion of the total current of the tube through either the plate circuit or the screen-grid circuit, according to the magnitude of the AGC voltage.

A comparison of curves A and B of FIGURE 8 with curves A and B of FIGURE 4 reveal the superiority of the amplifier of FIGURE 6 over the conventional amplifier of the prior art. Curve B of the FIGURE 8 substantially faithfully reproduces the form of the input signal and is free of the distortion displayed by curve B of FIGURE 4.

Since the grids of my novel tube are all preferably closely wound with a constant pitch, the capacitance between the anode and the control grid is very small. The electrodes 20, in addition to collecting the deflected electrons, also function partially as electrostatic shields between anode 18 and the supporting rods 13 of the control grid (see FIGURE 1). Thus, in an embodiment of my invention, the maximum capacitance between the anode and the control grid is only 0.005 micro-microfarads. In some sharp cut-off pentodes of the prior art the corresponding measurement may be as high as 0.03 micro-microfarads.

In addition to supporting the control grid, the rods 13 exert a focussing and narrowing effect on the stream of electrons flowing from the cathode to the anode. Thus the rods 13 tend to accentuate the influence of the deflecting means 19.

It will be understood, of course, that the anode of my novel tube need not be in two parts as illustrated in FIG- URES 1, 2 and 7. It may be in only one part or it may be in more than two parts. The number of main paths for the current may therefore be one or more depending 8 on the number of parts of the anode. However, the preferred number of parts is two, as illustrated in FIG- URES 1, 2 and 7.

Other modifications will occur to those skilled in the art. The' preferred embodiment of my novel tube as disclosed herein is intended primarily for use in television receivers, at frequencies in the neighborhood of 4S m.c./s. However, the principles disclosed herein could be applied to tubes used in other applications and at other frequencies.

What I claim is:

1. Electronic circuit means comprising an electron discharge device, said device comprising an evacuated envelope, said envelope enclosing a plurality of electrodes including: an electron-emissive cathode; a control grid, a screen grid, and a suppressor grid, said grids being closely wound with a constant pitch and spaced in succession from said cathode and from each other; an anode spaced from said suppressor grid on the side away from said cathode, said anode and said cathode defining therebetween at least one main path for electron emitted from said cathode, said path traversing said grids, and means to prevent electrons emitted by said cathode from returning to the region between the cathode and the control grid comprising an electron-deflecting deformation of the suppressor grid, said deformation comprising a generally V-shaped projecting portion directed toward said cathode and situated in said main path.

2. An electron circuit means as recited in claim 1 wherein said preventing means further includes an electron collecting means electrically connected to the screen grid whereby slow electrons deflected by the deformation of the suppressor grid are collected.

References Cited by the Examiner UNITED STATES PATENTS 2,146,016 2/1939 Herold 31369 X 2,219,102 10/ 1940 Herold 3 l369 2,572,055 10/1951 Saldarini 3136 X 2,726,353 12/1955 Wallmark 313-72 X 2,766,331 10/ 1956 Birkemeier 33013l ROY LAKE, Primary Examiner.

N. KAUFMAN, Assistant Examiner. 

1. ELECTRONIC CIRCUIT MEANS COMPRISING AN ELECTRON DISCHARGE DEVICE, SAID DEVICE COMPRISING AN EVACUATED ENVELOPE, SAID ENVELOPE ENCLOSING A PLURALITY OF ELECTRODES INCLUDING: AN ELECTRON-EMISSIVE CATHODE; A CONTROL GRID, A SCREEN GRID, AND A SUPPRESSOR GRID, SAID GRIDS BEING CLOSELY WOUND WITH A CONSTANT PITCH AND SPACED IN SUCCESSION FROM SAID CATHODE AND FROM EACH OTHER; AN ANODE SPACED FROM SAID SUPPRESSOR GRID ON THE SIDE AWAY FROM SAID CATHODE, SAID ANODE AND SAID CATHODE DEFINING THEREBETWEEN AT LEAST ONE MAIN PATH FOR ELECTRON EMITTED FROM SAID CATHODE, SAID PATH TRAVERSING SAID GRIDS, AND MEANS TO PREVENT ELECTRONS EMITTED BY SAID CATHODE FROM RETURNING TO THE REGION BETWEEN THE CATHODE AND THE CONTROL GRID COMPRISING AN ELECTRON-DEFLECTING DEFORMATION OF THE SUPPRESSOR GRID, SAID DEFORMATION COMPRISING A GENERALLY V-SHAPED PROJECTING PORTION DIRECTED TOWARD SAID CATHODE AND SITUATED IN SAID MAIN PATH. 